Abstract
We present a layout-aware optimization methodology for spin-transfer torque (STT) MRAMs, considering the dependence of cell area on the access transistor width (W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FET</sub> ), number of fingers in the access transistor and the metal pitch of bit- and source-lines. It is shown that for W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FET</sub> less than a critical value (~7 times the minimum feature length), one-finger transistor yields minimum cell area. For large W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FET</sub> , minimum cell area is achieved with a two-finger transistor. We also show that for a range of W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FET</sub> , the cell area is limited by the metal pitch of bit- and source-lines. As a result, in the metal pitch limited (MPL) region, W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FET</sub> can be increased with no change in the cell area. We analyze the impact of increase in W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FET</sub> in the MPL region on the write margin and cell tunneling magneto-resistance (CTMR) of different genres of STT MRAMs. We consider conventional STT MRAM cells in the standard and reverse-connected configurations and STT MRAMs with tilted magnetic anisotropy for the analysis. By increasing W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FET</sub> from the minimum to the maximum value in the MPL region (at iso-cell area) and reducing read voltage to achieve iso-read disturb margin, 2X improvement in write margin and 27% improvement in CTMR is achieved for the reverse-connected STT MRAM. Similar trends are observed for other STT MRAM cells.
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